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Not yet fully autonomous

Although the cars are promoted as "self-driving", they must constantly be monitored by an actual driver who can take control any time the software should fail.

Uber says its technology is not sophisticated enough to have cars continuously drive by themselves - and therefore the permit demanded in San Francisco is not needed.

Image copyrightUberImage caption
Uber began using Ford cars to test its self-driving technologies in Pittsburgh earlier this year

But regulators say that 20 other companies testing self-driving technology in California - including Google, Tesla and Ford - do so with the special permit which requires the firms to report any accidents to the authorities. This potentially undermines the company's reputation, as the reports are available to the public.

Just before the city's Department of Motor Vehicles threatened Uber with legal action over the missing permit, a video of an Uber self-driving vehicle running a red light in San Francisco had been uploaded to YouTube.

The trials are designed for the firm to identify flaws and glitches in the technology behind the autonomous driving.